The goal of this research is to quantify the performance of respirator filters used to protect workers in environments where they are at risk of exposure to asbestos and manmade fibers. Current OSHA guidelines for the respiratory protection of workers exposed to asbestos require the use of respirators fitted with high efficiency filters. These filters must be approved under the NIOSH certification criteria based on penetration tests using spherical aerosols. Fiber aerosols are known to have different aerodynamic behaviors than spherical particles and usually carry higher electrostatic charges. Because the carcinogenicity of asbestos and other fibers is known to be due, in part, to fiber dimensions, it is very important to know the efficiency of respirator filters in relation to fiber dimension. However, the limited data reported do not provide information on the effects of fiber size on filter performance. These are also inconsistencies in the reported data under different laboratory conditions. Based on these observations, it is difficult to predict how fiber aerosols will penetrate respirator filters based on the NIOSH testing results using spherical particles, nor can we assume that a filter suitable for a certain fiber aerosol is also good for other types of fibers. There are increasing needs to quantify filter performance for worker protection, because of the expanding asbestos abatement industry and the increasing use of manmade fibers as a substitute for asbestos fibers. The specific aim of this study is to elucidate the effects of fiber size, electrostatic charge, and flow rate on fiber aerosol penetration of filter cartridges. Three asbestos fibers with mean diameters from 0.04 to 0.5 mu m and aspect ratios from 3 to 60 will be used. Both overall and size- specific penetration of fiber aerosols through filters will be determined. Based on size-specific fiber penetration information, the effects of particle diameter, length, and flow rate will be examined and the filtration mechanisms identified. We will then determine the dimensions of fibers that are most likely to penetrate the respirator filter. We also hypothesize that electrostatic collection is the main reason for the inconsistent filter performance reported in the literature. This hypothesis will be tested by investigating the effects of electrostatic forces on fiber collection. Filtration models will be established to predict the penetration of fiber aerosols of given size distributions and electrostatic charges. Comparison of filter penetration efficiencies for spherical particles and fibers under these conditions will provide a scientific basis for evaluating the adequacy of current NIOSH guidelines regarding the respiratory protection for exposure to asbestos and its manmade substitutes, and for recommending possible improvements. The penetration of fiber aerosols through leaks in a respirator face seal will be measured. Also the effects of mass loading of fiber aerosols on flow resistance and fiber aerosol penetration will be determined. These studies can provide important information in determining the respiratory fit factors for field use.